As one technique for realizing an increase in the capacity of an optical communication network, wavelength division multiplexing (WDM) is widely used. According to WDM, a plurality of channels of different wavelengths are multiplexed.
In an optical network using WDM, an optical add-drop multiplexer (ROADM: Reconfigurable Optical Add-Drop Multiplexer) is provided to each optical node. The optical add-drop multiplexer is capable of separating an optical signal at a specified wavelength from a WDM optical signal and guiding the signal to a client, and of inserting a client signal generated by the client into the WDM optical signal. Accordingly, the optical add-drop multiplexer includes wavelength selective switch (WSS) that selectively passes an optical signal at a specified wavelength from a plurality of optical signals in the WDM optical signal.
The wavelength selective switch selects a channel at a specified wavelength. For this purpose, the wavelength selective switch includes a wavelength tunable optical filter. That is, the wavelength selective switch controls the transmission wavelength band of the wavelength tunable optical filter such that an optical signal at a specified wavelength is selected. However, if the transmission wavelength band of the wavelength tunable optical filter is not appropriately controlled, the quality of a selected optical signal is reduced. Thus, the optical add-drop multiplexer preferably includes a function of monitoring whether the transmission wavelength band of the wavelength tunable optical filter is appropriately controlled.
As a related technique, an apparatus for detecting a deviation of the wavelength transmission characteristics of a wavelength multiplexing filter with high accuracy is being proposed. This apparatus includes a signal selector, an input-side OCM (Optical Channel Monitor), an output-side OCM, and a detector. The signal selector performs wavelength multiplexing of optical signals. The input-side OCM acquires a spectrum of an optical signal to be input to the wavelength multiplexing filter. The output-side OCM acquires the spectrum of an optical signal output from the wavelength multiplexing filter. The detector detects a deviation of the wavelength transmission characteristics of the wavelength multiplexing filter based on the difference between the spectra acquired by the input-side OCM and the output-side OCM. (For example, Japanese Laid-Open Patent Publication No. 2011-254309)
As other related techniques, there are known Japanese Laid-Open Patent Publication No. 2004-364033, and “Distance-Adaptive Spectrum Resource Allocation in Spectrum-Sliced Elastic Optical Path Network”, Masahiko Jinno et. al., IEEE Communications Magazine, August 2010.
In recent years, there is a demand for a flexible optical network to flexibly cope with a change in traffic, unexpected update of a network configuration, and the like. In such an optical network, the bit rate and/or the modulation scheme of each optical channel is dependent on the required transmission capacity and transmission distance. That is, the spectrum bandwidths of the optical channels are not the same. Thus, to effectively use the limited resources (in this case, the wavelength), a WDM transmission system adopting a flexible grid is being put to practical use.
With a conventional fixed grid system, channels of WDM are arranged at predetermined intervals. On the other hand, with a flexible grid system, channels of WDM may be arranged at arbitrary wavelengths. Accordingly, in a WDM transmission system adopting the flexible grid, the transmission band of a wavelength tunable optical filter for selecting a channel at a specified wavelength from a WDM optical signal is required to be accurately controlled.